The present invention relates to a novel anti-reflective film useful for reducing stray light in various kinds of optical instruments or, more particularly, to an anti-reflective film which is attached to the inner wall of a photographic camera or other optical instruments with an object to decrease stray light arriving at the photographic film or photodetector means due to low-angle incident light through the camera lens.
As is known, optical instruments in general, such as photographic cameras, copying machines, developing machines and the like, have a problem of drawbacks due to the stray light by the irregular reflection of the incident light on the inner walls of the instrument. In a photographic camera, for example, the low-angle incident light through the lens is irregularly reflected on the inner surface of the camera and reaches the photographic film to cause undue sensitization of the film or to enter the optical path of the range finder decreasing the visibility of the view field images. Many of modern cameras are designed to be equipped with a zoom lens having variable focal lengths, in which an undesirable phenomenon of halation or ghost images appearing on the photograph as a result of irregular reflection of the low-angle incident light on the inner surface of the lens tube or on the surface of the flexible printed circuit board installed for driving the zoom lens or other systems of the camera resulting in intermixing of the stray light with the imaging light through the lens. In a copying machine, the photoimages formed in the photosensitive body is subject to a decrease in the sharpness and contrast due to the light produced inside the machine and reaching the surface of the photosensitive body as reflected on the inner walls of the machine. In a developing machine of photographic films as exposed, stray light formed by the reflection of leak light into the machine causes sensitization of the film under development or reaches the photographic paper under development resulting in a decrease in the contrast of the photograph as developed.
It is a conventional measure with an object to solve this problem that an optical instrument or, in particular, photographic camera is so designed that the camera body, shutter case, cover and the like are shaped from a black-colored plastic resin and imparted with a matted surface or, alternatively, a black flocked paper sheet is adhesively bonded to the surface of the printed circuit board built in the camera and inner walls of the optical instrument.
The above mentioned former method of forming the inside wall of an optical instrument from a black-colored plastic resin with a matting treatment of the surface is industrially and technically not practicable because the adverse effects of stray light due to irregular reflection can hardly be estimated with sufficient reliability at the stage of instrument designing so that the treatment of the surface on a particularly limited area can hardly be undertaken selectively unless a great increase in the manufacturing costs of the instruments is disregarded. What is worse in this method is that the anti-reflective effect obtained therewith cannot be high enough to the light incident at a low angle. The method employing a black-colored flocked paper sheet has disadvantages that the effective inside volume of the camera body and the like is necessarily decreased as a result of attaching the flocked paper sheet having a relatively large thickness on the inner surface of the camera body and that the flocks of the flocked paper sheet sometimes fall off the paper sheet to cause serious dustiness if not to mention the problem of susceptibility of the flocked paper sheet to deposition of dust particles thereon from ambience.
Along with the trend in recent years toward more and more compactness of photographic cameras and the like, various parts of the instrument are also required to be more compact so that the anti-reflective sheet to be attached to the inner surface of the instrument is also required to have a high anti-reflective effect with a thickness as small as possible and to be free from the problem of occurrence of dusts.